1. Field of the Invention
The present invention relates to a fixing unit for fixing a toner image to a sheet and an image forming apparatus with the built-in fixing unit.
2. Description of the Related Art
An image forming apparatus such as a copier, a facsimile machine or a printer typically includes a fixing unit configured to fix a toner image to a sheet. A certain type of fixing units includes a heating roller and a pressure roller to be pressed to the heating roller. While a sheet bearing a toner image passes through a nip portion between the heating roller and the pressure roller, the toner image is fixed to the sheet.
Another type of fixing units includes a heated belt and a pressure roller to be pressed to the belt. While a sheet passes through a nip portion between the belt and the pressure roller, a toner image is fixed to the sheet.
The aforementioned belt-type fixing unit includes a heating roller on which the belt is wound, and a fixing roller configured to nip the belt with the pressure roller, in addition to the above belt and pressure roller. The belt is wound on the heating roller and the fixing roller. The fixing unit includes a tension roller configured to maintain the tension of the belt. The tension roller stabilizes the belt tracking.
The aforementioned fixing unit includes a heating mechanism for induction-heating the belt and the heating roller. The tension roller is typically biased in a direction orthogonal to a travel path of the belt defined between the heating roller and the fixing roller.
The tension roller biased in the direction orthogonal to the travel path of the belt defined between the heating roller and the fixing roller changes a distance between the heating mechanism and the belt. For example, when the belt tension goes down, the tension roller is displaced in a direction away from a straight line connecting the rotation axes of the heating and fixing rollers. When the belt tension goes up, the tension roller is displaced in a direction toward the straight line connecting the rotation axes of the heating and fixing rollers.
The movements of the tension roller toward and away from the straight line connecting the rotation axes of the heating and fixing rollers change a distance between the belt and the heating mechanism at the start and/or end points of a heating zone defined by the heating mechanism. The change in the distance between the belt and the heating mechanism becomes a disturbance factor for a temperature control of the belt to eventually cause failures in a toner image fixing process.
The aforementioned failure is found not only in the induction heating type fixing units, but also commonly found in fixing units including another heating mechanism configured to supply heat energy to an outer surface of a belt.